California banana slugs: Fun facts about our vibrant, terrestrial molluscs

Editor's note: In 2024, the banana slug was named the California state slug by official decree. Learn more about this beloved slug below. 

By Will Urich
Park Stewardship San Mateo Intern

As a middle school student in San Mateo County, I vividly remember my first encounter with a banana slug. During a week-long outdoor education trip in Pescadero with my class, I had the opportunity to learn about how slugs play a vital role in forest and riparian ecosystems. Access to outdoor education opportunities in schools like this can help foster a love of nature and learning, and I’ll never forget how these experiences were a wonderful jumping off point for my current passion for the outdoors.

Hiking along a trail in Rancho Corral de Tierra last week, I came upon one of these gentle and pleasant creatures, which struck up a fond memory. If you’re ever walking in cooler and moisture-rich environments, such as by coniferous trees like the California coastal redwood or Monterey pine, keep your eye out for a banana slug and try to investigate its behavior and anatomy.

The California banana slug is the second largest slug in the world after the European Limax cinereoniger. It can grow up to 10 inches long, move about 6.5 inches per minute, and can have variations in coloration based upon diet, exposure to light and elements, and the moisture levels present in the atmosphere.

California banana slugs, like all slugs, are detritivores—they process leaves, animal droppings, moss and dead plant material for food and recycle the material into nutrient-packed soil humus. This humus gives the soil more nutrients, similar to compost.

One myth to debunk is to never pick up and kiss slugs when you find them in nature, even if you’ve heard that it can bring you good fortune. The slimy coating on slugs helps the slugs breathe and protects it from predators, producing a numbing, foul taste. The slime absorbs 100 times its weight in water from whatever it touches and transfers chemicals and nutrients readily through the mucus membrane into the slugs’ organs. This means that if you have sweaty or dirty hands, it can truly have a negative effect on the slug.

Fun fact: the slime coating on slugs is neither a liquid nor a solid, but rather a liquid crystal that is currently being studied by material scientists and has already been synthesized to aid in surgeries as an advanced, biodegradable glue used for suturing!

So next time you are out and about in our San Mateo parklands, take some time to look around and see if you can spot a banana slug. I hope you can appreciate these wonderful, cool and important members of our local ecosystems!